The present invention relates in general to an electromagnetic lifting device, and more particularly to an electromagnetic lifting device specifically designed for lifting up a single sheet of ferromagnetic metal such as steel sheet.
In the case that it is required, by using an electromagnet, to pick up and lift a sheet of steel material, sheet by sheet, from a pile of such steel materials stored at a material stock yard as in a sheet steel processing factory, it is most adaptable and efficient in practice to apply an electromagnet having a magnetic pole face area in accordance with a thickness of such sheet material. However, for this purpose, it is eventually essential to have a plurality of electromagnetic elements producing an electromagnetic energy that correspond to each of thicknesses of such sheet material in stock, and this is so inconvenient and impracticable.
In order to avoid such inconventineces, it is generally a practice that an electromagnet is prepared which is adaptable to magnetically hold and lift up a sheet material having a possibly largest thickness in stock or having a relatively large area of electromagnetic pole face, and then this particular electromagnet is used to hold and lift up a sheet material having a relatively thin thickness by adjusting an energizing power at an optimal level for handling such relatively thin sheet steel.
Particularly in the case that a sheet material having such a relatively thin thickness as 9 mm or less is handled with the electromagnet having such relatively large area of magnetic pole face, however, the energizing power may be restricted to a lowest possible extent, there is exerted an electromagnetic force not only to an uppermost sheet in a pile of sheet material but also to a next one or more sheets, thus resulting in such an inconvenience that one or more sheets of steel material might as well be induced to draw to each other and lifted upwardly together. This is because an excessive magnetic force is exerted to the uppermost sheet material and the successive lower sheet material in a piled stock.
In such case, such steps are generally taken that two or more sheets material that happen to draw to each other are once lifted upwardly together under the effect of the electromagnet, then the energizing power is cut off once for a very short moment by flipping a current switch to off so that the desired only one sheet material may be left drawn and rest on the magnetic pole face of the electromagnet as it is by function of a residual magnetism working thereon and the unwanted extra one or more sheets of steel induced to be attracted to the required one sheet may fall out of engagement therewith by their own weights down to the ground, thereafter having the energizing current restored in the energizing coil by putting the switch back to "ON" position so as to maintain the attraction or magnetic force of the electromagnet on the desired one sheet.
However, it is apparent that such manual switching operation is an unreliable and dangerous task and would turn to be a substantial loss in the working time even with a proficient operator, and also, it is impossible to remotely conduct such delicate operation at the working site. Moreover, this manual operation should naturally be practiced under the immediate supervision of the operator, and therefore, such lifting services may not be made under an unmanned or automated condition. In addition, as in the case of handling the steel sheet having a relatively long extension wherein a plurality of electromagnet are used to operate at a time to lift up a sheet steel of a long dimension, it is very difficult to have only one sheet of lengthy steel sheet kept drawn and rest stably in position of the magnetic pole face with a single operation because of an occasional difference or unevenness in the states of magnetic holding effected by each of these electromagnets.
How advantageous it would be if there is made available an efficient means to overcome such problems as encountered in the prior art electromagnetic lifting devices. The present invention is essentially directed to meet such requirements.